Sodium disorders

Low and high [Na] 25 mins.

Rob Hunter, Consultant Nephrologist. Part of Renal/Urology in the Edinburgh MBChB.

Hyponatraemia

Hyponatraemia is a problem of low plasma tonicity. Or you can consider it as a problem of excess body water.

Severe acute hyponatraemia is a medical emergency because it can cause life-threatening cerebral oedema.

In health, plasma tonicity is controlled in a negative-feedback loop in which anti-diuretic hormone (ADH) is released from the posterior pituitary in reponse to rising tonicity. ADH acts in the collecting ducts of the kidney, opposing the excretion of water in the urine.

Diagnosing the underlying cause is important in order to guide appropriate management. In addition to history and examination, the plasma osmolality (POsm), urine osmolality (UOsm) and urine sodium concentration (UNa) can help.

In general hyponatraemia may be caused by:

• translocation of water into the circulation such as in severe hyperglycaemia (in which case plasma tonicity will be normal and there is no risk of cerebral oedema; POsm > 280 mOsm/L)
• excessive water intake or low dietary solute intake (in which the urine will be very dilute: UOsm < 100 mOsm/L)
• release of ADH in response to hypovolaemia (in which case the renin-angiotensin system will also be activated so that urinary sodium excretion is low: UNa < 30 mM)
• release of ADH in response to what the body perceives as hypovolaemia in low cardiac-output or vasodilated states such as heart failure or portal hypertension (renin-angiotensin system will be activated so UNa < 30 mM)
• inappropriate release of ADH in euvolaemia in response to drugs, pain, stress or as a paraneoplastic pheomenon = SiADH (UNa > 30 mM)

Video (17 mins)

European Joint Societies algorithm

Click to enlarge

This is figure 6 from the very long full guideline.

Core materials

• Sodium disorders (Edren Textbook, shorter than this)
• NICE-CKS on hyponatraemia (UK-restricted?)
• Case(s) …

Hypernatraemia

Hypernatraemia is usually much easier to diagnose and to manage than hyponatraemia. The problem is a deficit in total body water. As thirst is such a powerful mechanism, this situation usually only arises in patients who have no access to water (e.g. in the perioperative setting). The treatment is to give enough water to replenish body water stores, accounting for any ongoing water losses. This may be given enterally (drinking or NG tube) or parenterally as intravenous 5% glucose.

Core materials

• Sodium disorders (Edren Textbook, shorter than this)
• Case(s) …

Further info

• Electrolyte disorders, a general approach (this site)
• More advanced … to follow, though you could start with the very long full hyponatraemia guideline